How can I use a bell to wake up a friend?

Some of your friends are walking on the territory of your map. You can explore different places with them and receive useful bonuses. When a friend helped you, he needs a day's rest before helping you again. You can use a bell in case you don't want to wait and go adventuring right away. Bells are given as a reward for assembling some collections and completing tasks. They can also be purchased in the store.